1. Field of the Invention
The present invention relates to a semiconductor process and in particular to a method for preventing sneakage in shallow trench isolation (STI) and structure thereof.
2. Description of the Related Art
The trend in developing semiconductor devices with smaller size and higher integration density has resulted in reduced distance between transistors. Shallow trench isolation (STI) is currently the most widely applied method isolating transistors. It has replaced the conventional method of local oxidation of silicon (LOCOS) to satisfy the requirements for generations less than 0.18 microns.
FIG. 1 is a cross section of a conventional STI structure, 14 represents a gate, 12 is an STI area, and n+ represents doped regions at two sides of the STI 12. Turning on a passing wordline (not shown) causes leakage of electric charge stored in the memory cell, a condition known as sneakage which is shown as 10 in the figure. This adversely affects performance of semiconductor elements. One current method of solving the above problem is forming STIs with deeper trenches. As trench depth shrinks, however, filling the trenches becomes more difficult.
Another method is implanting the sidewall of a trench to avoid formation of leaking passages therein. FIGS. 2A˜2C illustrate cross sections of the above-mentioned method. A semiconductor 16 having a nitride layer 20 and an oxide layer 18 formed thereon is provided, as shown in FIG. 2A. A photoresist layer 22 is used to define an isolation region, followed by etching to form a trench 24, shown in FIG. 2B. The sidewall of the trench is then implanted to form the implantation region 28, shown in FIG. 2C. Formation of a leaking passage on the sidewall is thus prevented. Finally, silicon oxide 26 is filled in the trench to form a shallow trench isolation structure.
The shortcoming of the above-mentioned method is damage caused to the semiconductor substrate during implantation. Hence, there is a need for a better method of avoiding sneakage in shallow trench isolation structures.